Language Learning Stack Exchange currently has sixteen questions about flash-cards, fifteen questions about anki and twenty questions about spaced-repetition. There is no doubt that spaced repetition is good for retention.
However, some people use paper flash cards, while others use digital flash cards. This leads me to the question whether there are any studies that show that paper flash cards are more effective for retention than digital flash cards (or the other way round). For the purpose of this question, it does not matter whether digital flash cards are online or stored locally. However, since making your own flash cards requires active involvement with the learning materials it probably has a learning effect. Hence, in order to compare like with like, studies should compare the effect on retention of self-made paper flash cards, i.e. written or created by hand, and self-made digital flash cards.
I found some discussion of this on Quora: Is learning with paper flashcards more effective than learning with digital flashcards?, but the answers there don't cite any studies. I would like to see studies that compare both types of media (paper versus digital) with regard to retention.